1. Field of the Invention
This invention relates to a method for molding digital storage memory cards such as, for example, multimedia cards (MMC), secure digital cards (SD), and similar small form factor digital memory cards.
2. Background Art
Thin form factor digital flash memory cards such as secure digital (SD), mini-SD, multimedia cards (MMC), and others contain three major components: an active flash memory device (including a flash IC and its controller); a printed circuit board (PCB) substrate; and an exterior cover that is typically manufactured from plastic. There are commonly two conventional methods of making these thin form factor digital memory cards.
The first method requires that semiconductor packages are surface mounted on the PCB substrate by way of a SMT production line. In this case, the active components (i.e., the flash memory and controller ICs) are located inside the semiconductor packages. These IC packages along with the necessary passive components (including surface mounted resistors and capacitors) form a complete subassembly that is known as a printed circuit board assembly (PCA). The finished card assembly is then completed by mating a plastic cover on top or two separate plastic covers on the top and bottom thereof in accordance with the standard design specifications. The thin cover top is typically molded plastic, but other materials (e.g., including a combination of metal and plastic) can also be employed.
The second conventional method for making a thin form factor digital flash card having a PCA subassembly is by means of a chip-on-board (COB). In the usual COB process, the active flash IC chips and their respective controller chips are mounted to the PCB substrate in a “chip and wire” process. That is, bare chips are assembled on the PCB substrate using wirebonds. However, the ICs are not assembled in a package as would otherwise occur in the aforementioned SMT production line assembly process. Nevertheless, when using the COB process, the supporting passive components may still need to be surface mounted to the substrate by means of an SMT assembly line. For a COB, the flash IC and its controller may be placed face up and a conventional wirebond (i.e., chip and wire) method is used to interconnect the I/O of the IC to the PCB substrate to thereby form the PCA subassembly. After the die attach and wirebond, it is usually necessary to apply an encapsulant over the bare flash and controller chips and their respective wires to provide suitable protection. This encapsulation step can be achieved by using either liquid encapsulants (globtop) or electronic molding compound (EMC) that are applied to the COB by means of a transfer mold. After encapsulation by either globtop or transfer mold, the finished PCA subassembly is then completed with an appropriate cover to achieve the final form factor for the intended memory card assembly.
It can be appreciated that for either of the two memory card assembly methods described above, the final step of making the finished card is attaching a (e.g., plastic) preformed cover. This cover can be a separate molded part and attached to the PCA by means of a thin layer of adhesive film. In the alternative, the cover can be bonded to the substrate of the PCA by ultrasonic fusion of the plastic material.
Alternatively, there is a more desirable method for forming and attaching the cover atop the PCA subassembly. This method includes injection molding directly over the PCA subassembly where the cover is formed in situ above the PCA packages. Therefore, there is no need to assemble a separate, preformed cover. This approach lowers the cost relative to requiring an adhesive or ultrasonic bonding of a separately preformed cover to the PCA subassembly.
What is more, injection molded parts have many advantages over a memory card assembly that requires a two part assembly process. In injection molding, the molded resin material will fill up all of the internal spaces inside the PCA subassembly. Thus, the finished memory card is mechanically more rigid and robust. Similarly, the card assembly will be more moisture resistant because there are no voids inside the card and no recessed grooves or large seams formed in the bond lines that are otherwise formed in the conventional two part (PCA and preformed cover) assembly. Injection molded parts also permit better dimensional stability and compliance to the design specifications of the memory card.
Even though such an injection molded method is more desirable and cost efficient, this method is presently applicable only to those PCAs that are manufactured by the COB process. The desirable injection molded method has not typically been applicable to PCAs that are made from the IC package/SMT process. The reason for this limitation is that IC packages having a commonly used TSOP (thin small outline package) format are thicker than the COB bare dice. Hence, any PCA using conventional TSOP packages would have a higher profile or body height than those made with the COB chip and wire process. A lower PCA assembly profile is necessary to permit injection molding such that sufficient resin material will flow into the narrow gap above the TSOP package while complying with the final finished memory card thickness requirement. When a conventional TSOP package is used to carry the flash IC in the PCA, the height of the TSOP package plus its substrate in the PCA is already approaching the maximum allowed overall thickness of the final memory card assembly. This leaves very little room for injection molding in areas directly above the TSOP package. Therefore, a PCA containing relatively tall packages, like the standard TSOP package, is unsuitable for injection molding. In other words, only COB PCAs having lower heights can be successfully used with an injection mold process.
Although the injection mold process to form an integral cover for the final memory card assembly is more direct and less costly, the conventional assembly steps of making a PCA by means of COB has other drawbacks which could reduce the advantage of the final injection mold step. One reason for some COB process drawbacks includes the requirement that the PCB substrates must have a soft gold surface finish on the wirebond pad area for gold wirebonding. This requirement increases the cost, because gold is plated onto the substrate and additional provisions must be provided to have plating bus lines running to the pads during such electric plating.
Another drawback is the result of the wirebond IC requiring protection by means of a liquid encapsulant in either globtop dispensing or transfer mold. This protection step requires additional time and cost. In addition, the cover is injection molded over the encapsulated PCA.
Yet another process drawback of COB is that the bare die that is mounted on the PCA typically cannot be fully tested and replaced, if necessary. After the assembly, if the flash IC or controller is found to be defective, the entire PCA subassembly will have to be discarded in its entirety.
Hence, there is a need for a manufacturing process by which to make a memory card assembly that can be used in high volume production with high yield and low assembly cost that is applicable to both a COB assembled PCA and to a TSOP assembled PCA. The injection mold material should be capable of fitting in the narrow space limitations above the TSOP package area while not causing damage to the connection of the TSOP package and the IC that is carried therewithin.